Conventionally, a honeycomb structure loaded with a catalyst is used for purifying harmful substances such as HC, CO and NOx contained in exhaust gas discharged from an engine of an automobile or the like. Thus, in the case of treating the exhaust gas with the catalyst loaded onto the honeycomb structure, the temperature of the catalyst needs to be raised to its activation temperature; however, since the catalyst has not reached the activation temperature at the time of starting the engine, there is a problem that the exhaust gas is not sufficiently purified. In particular, since plug-in hybrid vehicles (PHEV) and hybrid vehicles (HV) include running only with motors for their traveling, the engine starting frequency is low and the catalyst temperature at the time of starting the engine is low, and therefore, the performance for purifying exhaust gas tends to deteriorate.
In order to solve this problem, there has been proposed an electrically heated catalyst (EHC) in which electrodes are disposed in a honeycomb structure made of conductive ceramics, and the honeycomb structure itself is caused to generate heat by energization, whereby the catalyst can be heated to the activation temperature before starting the engine (see Patent Document 1).
In addition, there has also been proposed a heating device in which heat is transferred to the internal cell structure, by disposing a tubular resistance-heating type heater so as to surround the circumferential wall of the honeycomb structure and by heating the wall surface of the circumferential wall of the honeycomb structure, (see Patent Document 2).